1. Technical Field
The present invention relates generally to a device for controlling the polarization of an optical fiber and a method of controlling the polarization of an optical fiber using the same. More particularly, the present invention relates to a device for controlling the polarization of an optical fiber, which is capable of controlling both the amounts of and phase difference between the polarization components of light guided through an optical fiber, and a method of controlling the polarization of an optical fiber using the same.
2. Description of the Related Art
In many cases, the performance of communication using an optical fiber is dependent upon the polarization of light. Devices that are sensitive to polarization include various devices, including a wavelength multiplexer, a converter, a modulator, an amplifier and a receiver.
For example, in the case of a standard single mode optical fiber, the polarization state of light propagating through the optical fiber changes due to unavoidable thermal or mechanical stress during the manufacture thereof and the irregular sizes of some imperfect optical fiber cores. Furthermore, this change is also influenced by an external environment in which the optical fiber is located. Accordingly, the state of polarization is random and may change over time. This brings about a so-called polarization-mode dispersion (PMD) phenomenon that causes pulse dispersion and signal distortion that cannot be easily resolved while a signal is propagating through the optical fiber.
A device for controlling the polarization of an optical fiber is a device that controls the state of polarization so that a desired state can be achieved at the output end of an optical fiber using a controllable and reliable method in order to overcome the above-described disadvantages.
One of the main principles of a device for controlling the polarization of an optical fiber is to change the double refraction state of the material of an optical fiber. In general, double refraction refers to a phenomenon in which light is branched into two paths and propagates along the two paths inside a medium. This phenomenon occurs because a medium has different refractive indices depending on the polarization direction of light. A polarization component that propagates along a slow axis having a high refractive index has a slow phase speed and is thus phase-delayed, and a polarization component that propagates along a fast axis having a low refractive index has a high phase speed and is thus phase-advanced.
A medium configured such that the phase difference between two axes is maintained at a half wavelength is a half-wave phase delay plate, and a medium configured such that the phase difference between two axes is maintained at a ¼ wavelength is a ¼ wave phase delay plate. Double refraction is found in an optical fiber. Although double refraction may be caused by the asymmetry of an optical fiber core, a more important reason why double refraction occurs in an optical fiber is stress that is exerted on the optical fiber during the manufacture thereof.
An ideal device for controlling the polarization of an optical fiber prevents a coupling between two orthogonal polarization directions, thereby intentionally generating a double refraction pattern over the length of an optical fiber. The double refraction effect generated as described above is considerably higher than an indispensable polarization effect that causes a PMD effect, with the result that it becomes possible to perform control so that a desired polarization state is achieved at the output end of an optical fiber.
Furthermore, in order to generate high double refraction, a device for controlling the polarization of an optical fiber, which is capable of introducing high stress in a single direction based on the geometrical shape and material of an optical fiber, is desirable.
A conventional device for controlling the polarization of an optical fiber is constructed by applying the principle of a wave plate, that is, a phase delay plate. As illustrated in FIG. 8, an optical fiber coil 10′ corresponding to a half-wave phase delay plate is disposed at the center of a path between the input and output lines of an optical fiber, and coils 20′ and 30′ corresponding to ¼ wave phase delay plates are disposed beside the optical fiber coil 10′. During the process of bending an optical fiber so that the optical fiber forms a coil shape, stress is introduced.
As is well known, the ¼ wave phase delay plate converts arbitrary input polarized light into linear (plane) polarized light, and the half-wave phase delay plate can rotate arbitrary linear polarized light by a desired angle. Accordingly, when linear polarized light is passed through the ¼ wave phase delay plate, the half-wave phase delay plate and again the ¼ wavelength phase delay plate, the linear polarized light can be converted into any desired polarization state.
The conventional device applies tensional pressures having different magnitudes in a bending direction and a direction perpendicular to the bending direction by bending an optical fiber, thereby generating double refraction. This operation is based on the principle that the magnitude of double refraction varies depending on a bending curvature radius (the square of a curvature radius is inversely proportional to the difference in refractive index). The fact that the absolute value of the difference in double refraction is controlled means that the absolute value of the phase difference that is experienced by respective polarization components having passed through two axes, that is, slow and fast axes, can be controlled. U.S. Pat. No. 4,389,090 discloses a double refraction effect that is introduced into a coil-shaped optical fiber.
Another important principle of the device for controlling the polarization of an optical fiber is to control the amount of polarized light incident on slow and fast axes by controlling the angle of incident polarized light with respect to the double refraction axis of a medium.
Accordingly, when the angles of arrangement of coils are controlled by rotating three coils 10′, 20′ and 30′ by the angles in the directions of the arrows using an optical fiber path axis a′ as a reference axis in FIG. 8, the quantities of polarization components propagating along the two axes can be controlled under a fixed wavelength condition.
Meanwhile, according to the conventional technology, since the curvature radius of the coils that form loops is fixed, inconvenience is incurred in that another phase delay plate should be newly installed and the angles should be controlled again when an optical fiber having a different wavelength is employed.
Another conventional technology is a technology using a squeezer.
For example, U.S. Pat. No. 6,480,637 discloses a technology in which multiple high-precision grinding surfaces driven by piezoelectric elements are arranged and stress is introduced into an optical fiber by applying squeeze to the optical fiber. Wave plates are biased by 45° with respect to each other, and the delay of each wave plate component varies depending on the pressure of each optical fiber squeezer. However, this device is disadvantageous in that the durability thereof is poor, the volume thereof is large and the cost thereof is high.
Furthermore, a device commercialized as a Babinet-Soleil compensator is configured to generate linear double refraction by applying pressure to an optical fiber using an actuator via a squeezer rotating around the optical fiber and to fabricate an optical fiber wave plate having a delay factor that varies depending on a change in the pressure. However, in spite of the advantage of being applied to a wide variety of optical fibers, this device is disadvantageous in that the durability thereof is poor because the device employs a method of applying pressure directly to an optical fiber and in that it is difficult to control accuracy because a wide variety of optical fibers should be processed depending upon a single squeezer.